The St. Jude Medical (SJM) prosthesis is increasingly being used for cardiac valve replacement. In 1980, the manufacturer supplied 9,000 SJM prostheses to surgeons, and in 1983 this number had risen to 18,500. From 1977 through 1983, 45,000 SJM prostheses were supplied to surgeons. The SJM prosthesis appears to be durable, its orifice area is larger for its size than any other mechanical prosthesis or for any tissue valve mounted on a frame, its low profile prevents interference to leaflet movement by adjacent tissues, and it appears to be less destructive to blood elements than any other mechanical prosthesis. Its major disadvantage is its delicate hinge mechanism which potentially could be interferred with more easily than other mechanical prostheses by thrombus or suture or a calcific deposit. Additionally, like all mechanical prosthetic valves, anticoagulants are required. The SJM prosthesis is a bit more expensive than other presently available mechanical prostheses or bioprostheses. Recently, we examined the hearts of 2 patients who died early after replacement of the aortic valve with a SJM prosthesis and in each it appeared that movement of 1 of the 2 SJM prosthetic leaflets was interferred with by underlying severe hypertrophy and leftward positioning of the ventricular septum in 1 patient (who died 6 days after aortic valve replacement) or by underlying severely thickened anterior mitral leaflet in the other patient (who could not be separated from cardiopulmonary bypass). The most likely circumstance in which ventricular septum could prevent the application of adequate pressure being applied to the undersurface of a SJM leaflet in the aortic valve position is in the older patient with aortic valve stenosis.